The process of making an analog of botanic seed has been disclosed, e.g., in U.S. Pat. Nos. 4,562,663 and 4,779,376. As described therein, gel beads (or capsules) can be produced using various matrices. Among the disclosed matrix materials, alginate is considered particularly desirable because of its good gel-forming ability and economical price. However, when propagules (particularly somatic embryos, adventitious shoots, or shoot primordia) are embedded within alginate gel beads (obtained, for example, by dropping 2% sodium alginate solution containing plant propagules into 100 mM calcium chloride solution), it was observed that the propagules may emerge slowly from the bead, and some died within the bead without emergence. The reason for the poor emergence rate is believed to be the occasional low vigor of some of the propagules, which at times, was not sufficient for the propagules to emerge from the capsules. An additional limitation discovered relates to the physical characteristics of the matrix.
Alginate gel beads are also used in various commercial fields, such as in the food industry for confectionery or in the chemical industry for immobilizing carriers, because they can be prepared easily by dropping soluble alginate solutions, such as sodium alginate, into multivalent metal salt solutions, such as calcium chloride.
Certain of these uses are described in "Suiyousei Koubunshi, Mizu Bunsangata Jushi No Saishin Kako, Kaishitsugijutsu To Youto Kaihatsu Sogo Gijutsu Shiryoushuu" (Technique for Most Recent Processing, Quality Improvement and Application Development of Water Soluble Polymers and Water Dispersible Polymers), page 147-155, 1981, Keiei Kaihatsu Senta, Tokyo, Japan. In each commercial field, the merits for using alginate are indicated and some results in overcoming undesirable characteristics of alginate are reported.
For example, attempts have been made to change the undesirable texture of alginate for food applications or to stabilize and increase its low pressure-tolerance for long-term usage as immobilized carriers of chemicals. These improvements are classified into three types:
Type 1: Use of a special alginate formulation. PA0 Type 2: Use of a substance, singly or in combination with the complexing agent, which interferes with the reaction between alginate and the complexing agent, either during the complexing process or afterwards. Alternatively, complexing agents other than the widely used calcium salt can be used. PA0 Type 3: Addition of inactive substances (which create a discontinuous phase in the gel) to the alginate sol.
As an example of a Type 1 improvement, a method described in New Food Industry, Vol. 28, No. 4, p. 10 (1986) Shokuhin Shizai Kenkyukai, Tokyo, allows the production of a softer gel bead by using an alginate of high M/G ratio (ratio of D-mannuronic acid and L-guluronic acid).
An example of a Type 2 improvement is the use of a cation salt which does not complex sodium alginate. In Japanese Patent Application Laid-Open Publication No. 50868/1980, a softer alginate gel texture for human oral application was obtained by treating the alginate gel bead with monovalent cations after complexing with polyvalent metal ion. In Japanese Patent Application Laid-Open Publication No. 40051/1983, the gel texture for oral applications was improved by adding magnesium ion to the gel bead. In another example, Japanese Patent Application Laid-Open Publication No. 159759/1984, an improved method to make fish egg-like granules was demonstrated by adding glucono-delta-lactone into the complexing calcium solution.
To make harder immobilizing beads, an aluminum salt was used instead of calcium (Japanese Patent Application Laid-Open Publication Nos. 31991/1983 or 65185/1982). In U.S. Pat. No. 4,562,663 or Japanese Patent Application Laid-Open Publication Nos. 160885/1985, 216542/1984 or 74984/1984, methods are described in which a hard gel bead was obtained by treating the bead's surface with polymer cations, such as polyalkyleneimine, chitosan, or polyamino acid and its derivatives. Finally, in Japanese Patent Application Laid-Open Publication No. 10426/1977, zinc sulfate was used to improve alginate gel membranes for pack cosmetic material.
An example of a Type 3 improvement is disclosed in Japanese Patent Application Laid-Open Publication No. 12968/1976, in which alginate gels contain polymers which do not interact with alginate. A jelly with good texture was formed by adding natural pectin, locust bean gum and guar gum to alginate. In another example, Japanese Patent Application Laid-Open Publication No. 205496/1983, a microorganism-containing bead was formed by dropping a solution mixture containing microorganisms, sodium alginate and a polysaccharide having a carboxymethyl group into an aqueous complexing solution containing calcium ions or aluminum ions. In Japanese Patent Application Laid-Open Publication No. 39608/1983, an improved method for packing cosmetic materials by adding crystalline cellulose, filler, etc. to alginate sol was described.
For other purposes, there are examples of the use of specific complexing agents or the addition substances to the alginate sol. For example, Japanese Patent Application Laid-Open Publication No. 158708/1986, the supply of trace element was achieved by using sea water as complexing agent. In Japanese Patent Application Laid-Open Publication No. 40708/1986, an absorbent polymer was added to the alginate sol to improve water retention of the bead. In U.S. Pat. No. 3,688,437 and Japanese Patent Application Laid-Open Publication No. 262904/1987, embedded oxygen gas or air in the bead was used to supply oxygen to the seed. In Japanese Patent Application Laid-Open Publication No. 118103/1985, a method is described in which beads are complexed with biologically active substances, such as insecticides or fertilizers. In U.S. Pat. No. 4,583,320, the addition of auxiliary substances to gel beads is disclosed.
It is considered desirable to provide improved gel bead characteristics for botanic seed analog applications which include a self-breaking system in which the bead has a sufficient gel hardness for transport, handling and sowing, but becomes brittle and breaks open after sowing. To date, however, there are no known reports relating to botanic seed analogs or seed encapsulation which disclose improvements in the physical character of gel beads in regard to a self-breaking ability.